1. Field of the Invention
This invention relates to fuel cells and, in particular, to fuel cells which directly convert solid carbon fuels such as coal, petroleum coke, pyrolyzed biomass, and organic wastes into electricity. This invention further relates to fuel cells which utilize molten carbonate as an electrolyte.
2. Discussion of Related Art
Whereas a conventional fuel cell converts gaseous hydrogen-based fuels into electricity, a direct carbon fuel cell (DCFC) uses solid carbon fuel to electrochemically generate electricity. The reactions involved are:Anodic reaction: C+2CO3==3CO2+4e−Cathodic reaction: O2+2CO2+4e−=2CO3=Overall reaction: C+O2═CO2 
Like any fuel cell, direct carbon fuel cells are not limited by the Carnot cycle efficiency. The direct conversion of carbon to electricity is governed by the free energy change of the reaction. The conversion efficiency is the ratio of the free energy of the reaction at the operating temperature to the standard enthalpy:Thermodynamic efficiency=ΔGT/ΔH°298=ΔH−TΔS Since ΔS for the carbon oxidation reaction is nearly zero, the thermodynamic efficiency is nearly 100% and is nearly independent of temperature.
Another feature of the direct carbon fuel cell is that the activity of carbon, because it is solid, and the CO2 product, because it is undiluted, is unity and invariant. This allows complete utilization of the fuel. Furthermore, the CO2 product in the anode exhaust gas is pure and, therefore, may be readily disposed of or recovered.
Molten carbonate fuel cells are fuel cells comprising an anode electrode, a cathode electrode and a molten carbonate electrolyte, typically a combination of alkali carbonates, which is usually retained within a porous electrolyte matrix such as LiAlO2, disposed between the anode and cathode electrodes. These fuel cells operate at temperatures in the range of about 600° C. to about 700° C. at which temperatures the alkali carbonates form a highly conductive molten salt, with carbonate ions providing ionic conduction. At the high operating temperatures of molten carbonate fuel cells, Ni (anode) and nickel oxide (cathode) catalysts are adequate to promote reaction.
The Boudouard reaction is the redox reaction of a chemical equilibrium mixture of carbon monoxide and carbon dioxide in a given temperature. It is the disproportionation of carbon monoxide into carbon dioxide and graphite or its reverse2CO═CO2+CThe Boudouard reaction implies that at lower temperatures the equilibrium is on the exothermic carbon dioxide side and at higher temperatures the endothermic formation of carbon monoxide is the dominant product. For instance, in the high-temperature, reducing environment of a smokestack, carbon monoxide is the stable product. When the carbon monoxide reaches the cooler air at the top of the smokestack, the Boudouard reaction takes place, oxidizing the carbon monoxide to form carbon dioxide, and precipitating (reducing) the graphite to produce soot.
The use of molten carbonate fuel cells to directly convert carbon to electricity is known. See, for example, Cooper, J. F., “Direct Conversion of Coal-Derived Carbon in Fuel Cells”, Recent Trends in Fuel Science and Technology, ed. S. Basu, Springer, 2007, pp 248-266, and Cooper, J. F., “Direct Conversion of Coal-Derived Carbon in Fuel Cells”, UCRL-PROC-202196, 2004, pp 1-38. In these known fuel cells, CO2 gas is used to pneumatically introduce coal or other carbon material into the fuel cell. The CO2 is maintained at temperatures less than about 500° C. so that it does not react with the carbon fuel. The fuel cell is oriented in a slanting position and the carbon is immersed in molten carbonate. However, because the molten carbonate fuel cell matrix and cathode performance depend on an optimal carbonate filling based upon their pore structures and carbonate inventory, immersing the coal in a carbonate bath as taught by the Cooper reference would flood the matrix and the cathode, and, thus, yield poor performance. In addition, because the carbon is immersed in carbonate melt, the CO2 product in the anode must bubble through the melt to exit the fuel cell.